The present invention relates to a projection exposure apparatus used to transfer a mask pattern onto a photosensitive substrate in a photolithography process for producing, for example, semiconductor devices, image pickup devices (CCDs, etc.), liquid-crystal display devices, or thin-film magnetic heads.
To produce semiconductor devices, for example, a projection exposure apparatus is used to transfer a pattern formed on a reticle as a mask onto each shot area on a wafer coated with a photoresist. Hitherto, a step-and-repeat type (one-shot exposure type) reduction projection exposure apparatus (stepper) has frequently been used as a projection exposure apparatus for the pattern transfer process. On the other hand, to meet the demand that the area of a pattern to be transferred should be increased without substantially increasing the load on the projection optical system, attention has recently been paid to a step-and-scan type projection exposure apparatus wherein a reticle and a wafer are synchronously scanned relative to a projection optical system in a state where a part of a pattern on the reticle is projected as a reduced (demagnified) image on the wafer, thereby sequentially transferring the demagnified image of the reticle pattern onto each shot area on the wafer. The step-and-scan method has been developed by combining the advantage of the transfer method of the aligner (slit scan method) that transfers a pattern on the whole surface of a reticle onto the whole surface of a wafer in the magnification ratio of 1:1 by one scanning exposure with the advantage of the transfer method of the stepper.
In general, it is a requirement that resolution of projection exposure apparatuses be increased. One approach to increasing resolution is to use a light beam of a shorter wavelength as an illuminating light for exposure. Accordingly, use has recently been made of excimer laser light in the ultraviolet and far-ultraviolet regions as illuminating light for exposure, such as KrF excimer laser light (wavelength: 248 nm) or ArF excimer laser light (wavelength: 193 nm). The use of metal vapor laser light, higher harmonics of YAG laser light, etc. has also been examined.
When excimer laser light is used as illuminating light for exposure, for example, broad band laser light sources and narrow band laser light sources are available as excimer laser light sources. The term “narrow band laser light source” means a laser light source in which the spectral half-width of laser light is not more than 2 pm to 3 pm. The term “broad band laser light source” means a laser light source in which the spectral half-width of laser light is not less than 100 pm. When illuminating light of a short wavelength in the ultraviolet region or shorter wavelength region, such as excimer laser light, is used, vitreous materials usable for refracting lenses of projection optical systems are limited to materials such as quartz and fluorite. Therefore, as the wavelength of illuminating light used shortens as described above, it becomes more difficult to achromatize the projection optical system. Accordingly, it is desirable to use a narrow band laser light source in order to facilitate achromatization of the projection optical system.
However, the band of excimer laser light is originally broad. Therefore, in narrow band laser light sources, the oscillation spectral width of excimer laser light is narrowed by injection locking or the like. For this reason, the laser output of narrow band laser light sources is lower than that of broad band laser light sources. Further, narrow band laser light sources are inferior to broad band laser light sources in terms of lifetime and production cost. Therefore, in terms of the laser output, lifetime and production cost, broad band laser light sources are more advantageous than narrow band laser light sources. Accordingly, attempts have recently been made to use a broad band laser light source in a projection exposure apparatus having a projection optical system structured such that achromatization can be readily achieved.
Incidentally, projection optical systems usable in scanning exposure type projection exposure apparatuses (scanning projection exposure apparatuses) such as step-and-scan type projection exposure apparatuses include a catadioptric system that uses a concave mirror, and a refracting optical system formed from a combination of refracting lenses only, as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 6-132191. When such a catadioptric system is used, achromatization can be readily achieved by disposing a concave mirror in a group of refracting lenses because concave mirrors are free from chromatic aberrations. Consequently, it becomes possible to use a broad band laser light source, which is advantageous in terms of laser output, lifetime, etc.
Even in the case of using the second-mentioned refracting optical system, it is possible to use a broad band laser light source because the range of achromatization can be widened by increasing the proportion of fluorite in the entire refracting lens system.
In the above-described prior art, when the second-mentioned refracting optical system is used as a projection optical system, it is necessary to use fluorite for ten-odd lens elements in a total of twenty-odd lens elements, for example, in order to achieve achromatization over a wavelength width of the order of 100 pm to use a broad band laser light source. However, fluorite has the following properties: it is difficult to machine; the yield after the machining is unfavorably low; the change of refractive index with temperature is large; and the coefficient of thermal expansion is high, so that deformation occurs to a considerable extent in response to changes in temperature. Therefore, if many fluorite lenses are used, the temperature dependence of the image-forming characteristics of the projection optical system becomes unfavorably high.
When the first-mentioned catadioptric system is used as a projection optical system, it is possible to achieve achromatization over a wavelength width of the order of 100 pm by disposing a concave mirror in a predetermined position in a group of a predetermined number of refracting lenses, for example, because concave mirrors are free from chromatic aberrations. However, it is necessary in a scanning projection exposure apparatus to set the demagnification ratio for a pattern transferred from a reticle to a wafer on the order of from ¼ to ⅕, for example, and if a concave mirror is merely disposed in such a group of refracting lenses, the range in which favorable image-forming characteristics can be obtained becomes an arcuate area. Since the pattern areas on reticles have a rectangular external shape, if scanning exposure is carried out with such an arcuate area, the reticle scanning distance must be set to use considerably greater than the width of the pattern area. This causes the reticle-side stage to increase in size, unfavorably.
Further, when such an arcuate area is used, it is also necessary to use a lens that is not axially symmetric, and it is not easy to machine a non-axially symmetric lens with the desired accuracy.
Moreover, when a catadioptric system is used as a projection optical system, the projection optical system becomes large in size and complicated in arrangement because of routing of the image-forming light beam. Therefore, it is desirable to form the whole projection exposure apparatus in as compact a structure as possible by taking into consideration the reticle scanning direction, etc.
Further, it has been pointed out that when ultraviolet light such as excimer laser light is used as illuminating light for exposure, it is necessary to circulate nitrogen (N2) gas or a gas (e.g. air) having ozone removed therefrom in the projection exposure apparatus in consideration for the absorption of ultraviolet light by ozone and also the properties of photoresist. However, if all the gas in a chamber in which the projection exposure apparatus is installed is merely replaced by nitrogen gas or the like, for example, a problem arises in terms of worker safety during maintenance and the like.
Further, it is necessary in a projection exposure apparatus to control the amount of exposure light applied to a wafer according to the sensitivity, etc. of a photoresist used. In this regard, with a method wherein the amount of ultraviolet light emitted as pulsed light, such as excimer laser light, is reduced through an ND filter plate, for example, the ND filter plate may be damaged by intense pulsed light. Further, it is desirable that the amount of ultraviolet light should be capable of being controlled continuously and accurately. However, with the method wherein the amount of ultraviolet light is controlled by using a light-reducing plate such as an ND filter, the amount of light cannot always be continuously set, depending upon the positioning accuracy of the light-reducing plate.
Furthermore, in the conventional step-and-scan type projection exposure apparatus, the projection optical system and the reticle-side stage are secured to a column stood on a surface plate to which a stage (wafer stage) for holding a wafer is secured. Accordingly, when the reticle and wafer are synchronously scanned during exposure, vibrations of the stages affect the projection optical system, which should essentially be stationary. This may degrade the image-forming characteristics.
In general, conventional projection optical systems are refracting systems using spherical lenses or reflecting systems using spherical mirrors. Assuming that λ is the wavelength of exposure light (exposure light wavelength), NA is the numerical aperture of a projection optical system, and k is a process factor determined, for example, by a resist used, the line width of line-and-space patterns that can be transferred with high accuracy by a projection exposure apparatus is expressed byLine width=k·λ/NA  (1)
Recently, use has been made of KrF excimer laser light (wavelength: 248 nm) and even ArF excimer laser light (wavelength: 193 nm), and it has become possible to achieve a process factor k of the order of 0.45 by improving resists, applying high-resolution techniques, e.g. modified illumination, and utilizing a wafer planarization technique. The numerical aperture of the conventional projection optical systems is of the order of 0.6 at maximum. Substituting these values into Eq. (1) reveals that the minimum line width of patterns capable of being transferred by the conventional systems is about 150 nm as given by0.45·193/0.6≈145 (nm)
As stated above, the conventional one-shot transfer type projection exposure apparatuses (steppers) use as short an exposure light wavelength as 193 nm. However, because the numerical apertures of projection optical systems using spherical lenses or spherical mirrors are of the order of 0.6 at maximum, the minimum line width of transferable patterns is about 150 nm. As device patterns are becoming even finer, projection exposure apparatuses have recently been required to transfer line-and-space patterns of from 150 nm to 90 nm in line width with accuracy. In this case, if ArF excimer laser light (wavelength: 193 nm) is used as exposure light, the numerical aperture of the projection optical system must be further raised to a level of from 0.65 to 0.85.
However, if a projection optical system having a wide exposure area to effect one-shot transfer and further having a high numerical aperture is realized by using spherical lenses or spherical mirrors, the diameter of each indivisual lens becomes excessively large, giving rise to problems. Under these circumstances, two techniques have been proposed to increase the numerical aperture without causing the projection optical system to become so large in size. The first technique is a step-and-scan type projection exposure apparatus in which a reticle and a wafer are synchronously scanned relative to a projection optical system with a speed ratio corresponding to the projection magnification used, thereby sequentially transferring a pattern on the reticle onto the wafer. By introducing this technique and using a rectangular or arcuate slit-shaped exposure area elongated in the diametrical direction of the effective field of the projection optical system, it is possible to ensure an exposure field of a given width by using a projection optical system in which the diameter of the effective field is ½1/2 of that in a case where an approximately square exposure area is used in the one-shot transfer system.
According to the second technique, about 1 to 5 optical members (aspherical members), e.g. lenses, which have aspherized surfaces are inserted in a projection optical system. The use of the aspherical members makes it possible to shorten the overall distance, minimize the number of lenses required and reduce the lens diameter even when the numerical aperture of the projection optical system is increased. Vitreous materials usable in an extreme ultraviolet region of 200 nm or shorter are limited to quartz and fluorite in the present state of art. Fluorite has a relatively high production cost. Moreover, fluorite has a larger coefficient of linear expansion than quartz and is sensitive to variations in temperature. Therefore, it is desirable to minimize the number of fluorite lenses used by narrowing the wavelength band of exposure light as much as possible to reduce the load of achromatizing the projection optical system. By introducing this technique, it is possible to design and produce a practical projection optical system having a high numerical aperture and capable of effecting exposure over a wide exposure field.
However, the use of such an aspherical member involves a problem. That is, if the refractive index of a gas around the aspherical member changes in accordance with variations in the temperature, humidity and pressure (atmospheric pressure) of the environment of the projection optical system, distortion and other aberrations that are difficult to correct occur. This causes the image-forming characteristics to be deteriorated. In this regard, in the case of a spherical lens, the way in which aberrations change with variations in the environmental conditions can be predicted fairly accurately by simulation or the like. Therefore, it is possible to effect control such that predetermined image-forming characteristics are maintained, for example, by finely moving the position of the spherical lens. However, in the case of an aspherical member, the change of aberrations is complicated, and it becomes difficult to correct aberrations when the change of aberrations is large. Even if the amount of aberration change is small, the aberration correcting mechanism may become complicated.
Furthermore, there are cases where fluorite lenses are used to achieve achromatization to a certain extent depending upon the level of narrowing the wavelength band of exposure light. There are also cases where a large amount of fluorite is used as a vitreous material to cope with the situation that the wavelength of exposure light is becoming shorter. In these cases, because fluorite has a large coefficient of linear expansion, aberrations are likely to change to a considerable extent with variations in temperature.
In view of the above-described circumstances, a first object of the present invention is to provide a scanning projection exposure apparatus capable of using as a projection optical system a catadioptric system formed from a combination of refracting lenses which are all axially symmetric and a reflecting optical member, and also capable of obtaining favorable image-forming characteristics.
A second object of the present invention is to provide a projection exposure apparatus designed so that the whole structure of the apparatus can be made compact even when a projection optical system is employed consisting essentially of the above-described catadioptric system.
A third object of the present invention is to provide a projection exposure apparatus designed so that during exposure, the absorption of illuminating light for exposure is minimized, and during maintenance, the safety of workmen can be ensured.
A fourth object of the present invention is to provide a projection exposure apparatus using ultraviolet light as illuminating light for exposure and capable of accurately controlling the amount of ultraviolet light for exposure at all times.
A fifth object of the present invention is to provide a projection exposure apparatus capable of obtaining favorable image-forming characteristics independently of vibrations caused by the synchronous scanning of a reticle and a wafer.
A sixth object of the present invention is to provide a projection exposure apparatus having a projection optical system capable of obtaining high image-forming characteristics.
A seventh object of the present invention is to provide a projection exposure apparatus having a projection optical system capable of obtaining high image-forming characteristics and capable of maintaining the high image-forming characteristics independently of variations in environmental conditions such as atmospheric pressure.